1. Field of the Invention
The present invention relates to an image forming apparatus by electro-photography, such as an electro-photographic printer, copy machine or facsimile etc.
2. Description of the Related Art
In general, an image forming apparatus such as printer, copy machine or facsimile, is expected to provide a clear and fair printing as a matter of course.
And, hereafter, an image forming apparatus by electro-photography, is analyzed in view of providing such printing. Although, FIG. 2 shows the structure of present invention, this is useful for describing general structure and function of electro-photographic apparatus.
In this kind of apparatus, as shown in FIG. 2, a photo-sensitive drum 1 is charged negative by a charging roller 22 at first. And, next, beams of light are radiated on the surface of the photo-sensitive drum 1 by an LED exposer 5. Then, a certain latent image of static electricity is formed thereon. And, here, a toner image is formed on this latent image with toner, which is given from the surface of developing roller 7 to the surface of photo-sensitive drum 1.
In this occasion, the thickness of toner layer on the surface of developing roller 7, must be a certain adequate height, in order to form fair toner image on the surface of photo-sensitive drum 1. Here, the toner on the surface of developing roller 7, is supplied by the toner conveying roller 9 with a control of developing blade 21. And, a certain adequate voltage is impressed between the developing roller 7 and the toner conveying roller 9 with developing blade 21.
The toner image formed on the surface of photo-sensitive drum 1, is transferred onto the surface of paper 26 by a transfer device, comprising a transfer belt 25, a transfer roller 27 etc. And, the toner remained on the surface of photo-sensitive drum 1, is cleaned away by a cleaning device 23.
Moreover, as for color electro-photographic printer, as many as four image forming apparatuses, each of which comprises the same as mentioned above, are provided for four fundamental color of Y,M,C,K. And, in this kind of color printer, the each amount of toner transferred onto paper must be controlled more precisely than the monochrome printer mentioned above, in order to reproduce a fair color image, because four independent image of Y,M,C,K, are laid over with each other on the same surface of paper. Therefore, in this kind of color printer, adopted is a method of process control as follows.
That is, a certain patch pattern image is printed on the transfer belt 25 in advance. And, its color density is measured by the density sensor 16. Then, a condition of process control is decided according to the density data.
In this method, the amount of toner transferred onto paper is detected by the density sensor as well. And, the amount of toner sticking to paper is pre-estimated. Then, the voltage impressed between developing roller 7 and toner conveying roller 9 etc. is controlled according to this pre-estimated amount of toner. And, in this method, the voltage is decided in the consideration that a certain adequate amount of toner is conveyed from the conveying roller to the developing roller.
However, there are other points to consider, in order to obtain a clear and fair printing.
One of these points is the quality of toner, such as flowing ability, charging ability etc. These can remarkably change when printing operation repeated and the apparatus became old. For example, the flowing ability of toner decreases when the toner deteriorated with heat. This kind of heat is emitted with friction of the developing roller etc. Or, this kind of heat is conducted from a fixer of toner. On the other hand, the charging ability of toner can increase when the toner became old. This kind of situation can be caused especially in the occasion when low printing duty images are printed repeatedly, because the consumption of toner is less than usual and toner is remained in the tank to deteriorate. As a result, the amount of toner, which sticks to the latent image of static electricity, differs even if the same amount of toner is supplied to the surface of developing roller 7.
Another point is the quality of apparatus. For example, conveying ability of toner conveying roller can decrease with wearing away of the surface of the roller.
Therefore, it is necessary to consider changes of image forming apparatus itself after it formed many images. And, it is also necessary to consider temperature or humidity around the apparatus in order to watch the quality of toner.